# Energy Of A Single Photon In Em Radiation?

Energy Of A "Single" Photon In Em Radiation?

Is the energy of all photons in em radiation same? That is, say light differs from radio waves only in the number of photons per second

## Answers and Replies

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analogdesign
Science Advisor
No, the energy of a photon is proportional to its frequency. That is one reason why gamma radiation is dangerous but radio waves are harmless.

How can a single photon have frequency? Imagining a single photon travelling sinusoidally(?) it does not make sense to think that its energy has got anything to do with the sine wave. How can its energy vary just depending on its path?

Nugatory
Mentor
How can a single photon have frequency? Imagining a single photon travelling sinusoidally(?) it does not make sense to think that its energy has got anything to do with the sine wave. How can its energy vary just depending on its path?
It may not make sense, but that's still the way the world works. The most convincing evidence of this was discovered around the end of the 19th century (google for "photoelectric effect einstein").

EDIT: The photon most certainly does not travel a sinusoidal path. But it still has a frequency and that frequency is proportional to its energy.

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jtbell
Mentor
Imagining a single photon travelling sinusoidally(?)
No, photons do not travel in sinusoidal paths. Neither do electrons or other particles, which also have a "wave function" associated with them. There is no generally accepted answer to what this wave function "really really is," with the result that half the threads in our Quantum Physics forum are related to this puzzle. All we know for sure is that we can define this wave function, and do certain mathematical operations on it to predict the results of experiments very successfully, in a statistical sense at least.

1.Imagining some kind of wavy pattern are there 2 streams of photons moving perpendicular to each other?or is it merely after hitting an electron that it induces E and B?

2.Why would photons move in wavy pattern at all,cant they move in a straight line?

ZapperZ
Staff Emeritus
Science Advisor
Education Advisor
1.Imagining some kind of wavy pattern are there 2 streams of photons moving perpendicular to each other?or is it merely after hitting an electron that it induces E and B?

2.Why would photons move in wavy pattern at all,cant they move in a straight line?
Did you even read the post right before this by jtbell? Only you think that photons move in a "wavy pattern".

Zz.

f95toli
Science Advisor
Gold Member
Photons do not move in a "wavy pattern". That said, we can't really say anything about HOW they move. Photons are fundamentally quantum mechanical "objects" and it is not possible build up any inutition about how they "really" move, or even what they are (the same can be said about just about everything else as well).

We have extremelly good mathematical models, so the problem is NOT that we do not understand what photons are. But what these models describe is so different from our everyday macroscopic world that it is extemely difficult to get any inuitive grasp of them

Photons do not move in a "wavy pattern". That said, we can't really say anything about HOW they move. Photons are fundamentally quantum mechanical "objects" and it is not possible build up any inutition about how they "really" move, or even what they are (the same can be said about just about everything else as well).

We have extremelly good mathematical models, so the problem is NOT that we do not understand what photons are. But what these models describe is so different from our everyday macroscopic world that it is extemely difficult to get any inuitive grasp of them
I was having that picture of perpendicular electric and magnetic fields in mind...what do they actually mean?

Cant we make a common sensical assumption that they( or the wavefunction that represents it) move in a straight line.

Did you even read the post right before this by jtbell? Only you think that photons move in a "wavy pattern".

Zz.
I am sorry if had hurt your sentiments regarding photon...

Dale
Mentor
Cant we make a common sensical assumption that they( or the wavefunction that represents it) move in a straight line.
No. There are many instances where that can be experimentally demonstrated to be false. Even a single slit experiment will get photons travelling in bent lines, and two slit or gradient experiments can show that a single photon travels along multiple paths.

1 person
ZapperZ
Staff Emeritus
Science Advisor
Education Advisor
I am sorry if had hurt your sentiments regarding photon...
(Scratches head)

Er..... Alright then!

But you still didn't answer the question. It is hard to know if you just didn't read the responses you've been given, or you didn't understand what you read. It has nothing to do with "hurt sentiments", whatever those are.

Zz.

But you still didn't answer the question. It is hard to know if you just didn't read the responses you've been given, or you didn't understand what you read. It has nothing to do with "hurt sentiments", whatever those are.

Zz.
Yes I have read it.I made the mistake of thinking of frequency as only the number of particles passing through a point for a while.

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What does that common representation of em wave as perpendicular electric and magnetic field imply?

Is there any particular order of emission that is radio waves, microwaves etc in that order or are they emitted randomly?

jtbell
Mentor
What does that common representation of em wave as perpendicular electric and magnetic field imply?
It means that if you "shine" an electromagnetic wave on (say) an electron, it exerts electric and magnetic forces on the electron.

Is there any particular order of emission that is radio waves, microwaves etc in that order or are they emitted randomly?
What do you mean by "order of emission?"

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Dale
Mentor
What does that common representation of em wave as perpendicular electric and magnetic field imply?
Classically an EM wave is a propagating EM field in which the electric and magnetic fields are mutually perpendicular and perpendicular to the direction of propagation. That is the classical picture, but you really shouldn't mix the classical and quantum pictures.

Is there any particular order of emission that is radio waves, microwaves etc in that order or are they emitted randomly?
I don't know what you are asking here. Radio waves are low frequency, microwaves are slightly higher, infrared is higher than that, then visible light, ultraviolet, x-rays, and gamma-rays. Their emission is not random, but the way you phrased the question is confusing.

1 person
So there is only one stream of photons and no perpendicular streams ?
What I meant by order of emission is, is there some process sequentially producing photons of different frequency according to em spectrum? or are photons of different frequencies being produced at random?

Dale
Mentor
What I meant by order of emission is, is there some process sequentially producing photons of different frequency according to em spectrum?
You could do a frequency sweep, but this would typically be done with RF where the quantum mechanical description (photons) is not terribly helpful and you are generally better sticking with a classical description.

My personal recommendation is that it seems like you should learn classical EM before attempting to learn quantum electrodynamics.

or are photons of different frequencies being produced at random?
It might help if you identified the system you are interested in. The mechanism for producing photons is different if you are talking about a radio antenna, an incandesent light, an x-ray tube, or a flourescent substance.

jfizzix
Science Advisor
Gold Member
A photon is a hard thing to pin down (I'm a grad student in quantum optics, so it's something I think about a lot).

A photon follows most of the same quantum rules that other quantum particles do, and in that sense, it's just as hard to talk about a single photon as it is a single electron, or any other particle.

We can only infer properties about photons through the experiments we perform (like everything else). Photons are especially hard to discuss because most of the time, by measuring a photon, you destroy it (i.e. it gets absorbed by a detector).

The full-blown mathematical treatment of a photon (in my field) describes it as a particular kind of quantum state of the electromagnetic field. It's a very low energy state with only one photon in the field, but it is more than the vacuum. You can have 2-photon states, 3-photon states, coherent states, thermal states, entangled states, and many more yet to be thought of.

You can of course make approximations, which is why we don't need quantum field theory to design mirrors and lenses and such (yet). Almost all of the time (when you see quadrillions of photons) Classical Electromagnetism will describe everything just fine (easier said than done).

1 person
jfizzix
Science Advisor
Gold Member
The photoelectric effect is a simple experiment explaining how photons can have an energy proportional to their frequency, provided one is okay with light being divided up into chunks known as photons.

That being the case, scientists have shown that with high frequency light, you can knock electrons off pieces of metal. The brighter that light (the more photons), the more electrons get knocked off, and we can measure these electrons hitting detectors.

However, at low frequencies, no electrons get knocked off the metal, no matter how bright the light is.

We know that if the photon has enough energy, it can knock an electron off, and it can't if it doesn't.

More importantly, we can measure the kinetic energy of the electrons that get knocked off and see that the higher the frequency of the light, the higher the kinetic energy of the ejected electrons.

This relationship between electron energy and frequency of the incident light is precisely what we expect if we say the energy of a photon is proportional to the frequency of the light it's a part of.

It's hard to think of photons on their own, but if a photon is just a very tiny part of a beam of light, then it makes sense to speak of the frequency of a photon as the frequency of the light it is part of.

You could do a frequency sweep, but this would typically be done with RF where the quantum mechanical description (photons) is not terribly helpful and you are generally better sticking with a classical description.

My personal recommendation is that it seems like you should learn classical EM before attempting to learn quantum electrodynamics.

It might help if you identified the system you are interested in. The mechanism for producing photons is different if you are talking about a radio antenna, an incandesent light, an x-ray tube, or a flourescent substance.
I am talking in the context of sun

Drakkith
Staff Emeritus
Science Advisor
So there is only one stream of photons and no perpendicular streams ?
Ignore photons for now. I'll try to give you a very brief idea of what an EM wave is.

Classically, an EM wave is an oscillation of the electric and magnetic field vectors. A vector is a direction. This means that as an EM wave passes over an antenna, it will cause the electrons in that antenna to move one way and then the other as the phase of the wave changes. If you were to draw an arrow to represent the force exerted on the electrons, it would alternate between "up" and "down". (Or whatever two directions the wave was oscillating in)

Note that this oscillation is NOT in the direction of propagation. Imagine a water wave. The wave moves up and down, but it propagates outward, in a direction perpendicular to the oscillations. EM waves behave the same. They move outwards in a direction that is always 90 degrees, aka perpendicular, to the directions of oscillations. So you can have the electric field vector oscillating "up and down", the magnetic field vector "left and right", and the wave would move forwards.

Well, that's the gist at least. It's not 100% accurate, but it should help you understand what an EM wave is.

sophiecentaur
Science Advisor
Gold Member
I am talking in the context of sun
I would recommend that you read around a lot more about EM waves before you try to tie them in with the idea of Photons. Your questions and comments are all very random and disjointed and I have a feeling you are jumping into this subject far too deep, to start with. Your present 'picture' of photons is really not leading you anywhere useful and asking questions based on this picture seems just to be leading you further into confusion.
You will not be able to 'bend' Physics to fit your ideas - it has to work the other way round.
Do some reading and then come back to the thread.

1 person
Dale
Mentor
I am talking in the context of sun
The sun is a plasma, meaning that it consists of a bunch of charged particles. Because the particles are charged they are constantly emitting and absorbing EM radiation as they accelerate.